Cisco Cookbook

Chapter 22. Hot Standby Router Protocol

22.0. Introduction

Hot Standby Router Protocol (HSRP) is a Cisco proprietary standard that allows a router on a LAN segment to automatically take over if another one fails. It was developed to solve a common problem in shared networks such as Ethernet or Token Ring. The devices on these shared network segments are usually configured with a single default gateway address that points to the router that connects to the rest of the network. The problem is that even if there is a second router on the segment that is also capable of being the default gateway, the end devices don’t know about it. Therefore, if the first default gateway router fails, the network stops working.

Many methods for addressing this problem have come and gone over the years. The most obvious and most seriously flawed solution is to have the end users reconfigure the default gateway address in their workstations. This is a terrible solution for several reasons. There is a large chance of typographical errors: the conversion is slow, laborious, and often requires a reboot of the workstation; it relies on users noticing the problem in a timely manner, and it is unlikely that anyone will bother changing the address back when the original router recovers; it also requires that a human is handy to make the change, but devices such as printers and servers don’t usually have anyone sitting beside them when problems appear.

A slightly better solution that many organizations have used ...